Many of the most current cellular phones are provided with a microphone unit and a speaker unit that are joined by a hinge so as to be unfolded in use and folded not in use. To increase the convenience of such a foldable telephone set, a hinge opening/closing device is disclosed in Japanese patent laid open publication 8-125725 that allows the two parts of the telephone set to be held in the closed or folded state in a stable manner and to be readily unfolded or opened using only one hand. In this hinge opening/closing device, a torsion coil spring is used for providing the rotational drive power to the hinge device.
However, the torsion coil spring stores the maximum amount of rotational energy when the two parts are fully folded, and this energy is released rapidly when a pushbutton is pressed to unfold the two parts so that the resulting reaction could cause a springy motion to the telephone set. Also, the retaining force that keeps the telephone set folded is related to the force required to operate the pushbutton. Therefore, when the retaining force is increased, the force required to operate the pushbutton increases, and this impairs the handling of the telephone set.
Such an inconvenience could be avoided by providing a suitable amount of frictional resistance by using a damper mechanism or weakening the spring force. However, in either case, it is difficult to achieve a suitable initial operating speed, an adequately small operating force and a smooth operation over the entire stroke of operation.